This invention relates to a method and apparatus for plunger replacement in a press molding machine for molding glass products and, more particularly, for CRT valves.
Hitherto, there has been known a molding machine of the type having a plurality of molding tools arranged in equally spaced relation in a peripheral area of a rotary table adapted to be rotated intermittently so that when the intermittently rotating table is not in rotation, glass products are press molded by a plurality of plungers (as disclosed in, for example, Japanese Patent Laid Open Publication Nos. 59-3024 and 60-137838).
According to this prior art arrangement, glass products can be molded with the one operation in same number as the number of plungers, but it is necessary for the plungers to be individually replaced due to damage (surface roughening) or the like caused on the surface of the plunger.
Where the glass products are the CRT valves, the portion of each product which is molded by a plunger surface is required to meet high surface and dimension standards consistent for use as an image forming surface. Therefore, once damage is caused to the surface of any of the plungers, the plunger must be replaced immediately.
Conventionally, in a molding machine equipped with a pluraltiy of plungers, it is necessary for the operation of the molding machine to be stopped each time for replacing even only one plunger.
With such a molding machine, one difficulty is that during the process of such a plunger replacement, normal plungers having no damage have to be put out of molding operation. Another difficulty is that during the process of such replacement, temperature drop is inevitable which results in changes in the condition of temperature distribution the change in temperature distribution condition being, the most critical condition of all pressing conditions involved. A change in temperature distribution condition results in further difficulty such that when operation is resumed, considerable time is required until proper condition of temperature distribution is restored, so that in the interim period, the operation results in continued production of defective goods.
This invention is directed to overcoming the above-mentioned difficulties, and accordingly, it is an object of the invention to provide a method for plunger replacement which can minimize possible drop in productivity during plunger replacement and wherein plunger replacement can be performed without any change being caused to the condition of temperature distribution with respect to normal plungers, and an apparatus for use in carrying out the method.